The invention relates to a recording disc rotary drive control apparatus, and more particularly to a video disc rotary drive control apparatus wherein data is written on the video disc with a nearly constant packing line density.
FIG. 1 shows a conventional recording disc rotary drive control apparatus. In FIG. 1, there is shown a CLV (Constant Linear Velocity) disc 1 onto which data is spirally recorded at a constant linear speed and the disc 1 is driven by a spindle motor 2. The data stored on the disc is ready by a pickup 3 located thereabove and, as the disc 1 is played, the pickup 3 is moved linearly by a feed mechanism using a slider, etc. (not shown) in the radial direction thereof.
The output signal of the pickup 3 is applied to a signal processing circuit 4 for correction and the like. A playback synchronizing signal extracting circuit 5 is connected to the output of the signal processing circuit 4. The playback synchronizing signal extracting circuit 5 separates and extracts only a playback synchronizing signal from the output signal of the signal processing circuit 4 and supplies the extracted signal to a control signal generating circuit 6. The control signal generating circuit 6, for instance, compares the phases of the playback synchronizing signal and a reference synchronizing signal and produces a control signal representing the comparison result. A driving circuit 8 for driving the motor 2 is connected to the control signal generating circuit 6 through a changeover switch 7.
The position of the pickup 3 is detected by a position detector 9. The position detector 9 produces a position detecting signal at a level proportional to the distance covered by the pickup 3 in moving from the reference position. A voltage divider 10 is connected to the position detector 9, and a set voltage generating circuit 11 is connected to the output of the divider 10. The set voltage generating circuit 11, for instance, is composed of a voltage divider for dividing the output voltage of the divider 10. The output of the set voltage generating circuit 11 is connected to one input of a comparator 12. A frequency generator 13, coupled to the rotary shaft of the motor 2, produces pulses at a frequency proportional to the rate of rotation of the motor 2. A frequency-to-voltage converter 14 is connected to the output of the frequency generator 13, and the output voltage of the frequency-to-voltage converter 14 applied to the other input of the comparator 12.
The comparator compares the output voltage of the set voltage generating circuit 11 with that of the frequency-to-voltage converter 14 and drives the changeover switch 7 depending on the state of the output signal of the comparator. The changeover switch 7 normally relays an accelerating voltage V.sub.x to the driving circuit 8 and the control signal produced by the control signal generating circuit 6 thereto while it is driven.
In the conventional recording disc rotary drive control apparatus thus constructed, the accelerating voltage V.sub.x is supplied to the driving circuit 8 through the changeover switch 7 at the time the motor 2 is started and while the rate of rotation of the motor 2 is increasing.
A voltage corresponding to the distance covered by the pickup 3 in moving from the reference position in the radial direction thereof is produced by the position detector 9. This voltage is converted by the divider 10 into a voltage representing the reciprocal of the distance covered.
In this case, given that the rate of rotation of a disc being played at a linear velocity of v is w, at a pickup position a distance r from the center of the record, the relation v=rw is formed. Assuming that the linear velocity is constant, the distance r is inversely proportional to the rate of rotation w. Accordingly, since the output voltage of the position detector 9 is proportional to the distance r, the output voltage of the divider 10 is 1/r, that is, proportional to the rate of rotation w. The output voltage of the divider 10 is made to be proportional to the rate of rotation of the motor 2, enabling the playback synchronizing signal extracting circuit 5 to extract the playback synchronizing signal from the output signal of the signal processing circuit 4 in the set voltage generating circuit 11. For instance, as shown in FIG. 2, the output voltage (shown by a solid line) of the voltage divider 10 is divided at a predetermined ratio to become a set voltage (shown by a broken line and, given that the output voltage of the divider 10 at the position a of the pickup 3 is V.sub.a and its set voltage is V.sub.b whereas the output voltage of the divider 10 at the position b on the side closer to the periphery than the position a is V.sub.a ' and its set voltage is V.sub.b ', the relation V.sub.b /V.sub.a =V.sub.b '/V.sub.a ' is satisfied.
The pulse signal produced by the frequency generator 13 at a frequency proportional to the rate of rotation of the motor 2 is converted by the frequency-tovoltage converter 14 into a voltage proportional to the frequency at which the pulse is produced. The voltage thus converted is compared with the set voltage in the comparator 12.
When the voltage converted is smaller than the set voltage, the accelerating voltage V.sub.x is, as mentioned above, supplied to the driving circuit 8 in an attempt to increase the rate of rotation of the motor 2. If the voltage converted is greater than the set voltage, the output level of the comparator 12 will be inverted from the low to the high level. In response to the high level supplied to the changeover switch 7 as a switching signal, the changeover switch 7 is switched over so that the control signal produced from the control signal generating circuit 6 is applied to the driving circuit 8. Accordingly, the rate of rotation of the motor 2 is loop-controlled in response to the control signal based on the synchronizing signal.
With the above-described arrangement, playback is made possible from any position on the disc, despite a difference in the rate of rotation of the motor 2 depending on the pickup position.
However, the conventional rotary drive control apparatus is disadvantageous in that it requires a costly voltage divider having a complicated circuit configuration. Moreover, high conversion accuracy is needed for the frequency-to-voltage converter, thus making the circuit of the apparatus as a whole complicated and expensive.